Hypodermic syringes are used to inject or extract liquid solutions from body tissues. Pain experienced by patients using hypodermic syringes continues to be a problem, and is a primary cause of missed drug administrations and appointments. Smaller gauge needles have been introduced in an effort to reduce pain associated with penetrating skin surfaces with needles. However, smaller gauge needles tend to clog when being used to administer viscous drug solutions or suspensions, including suspensions containing microspheres. Larger gauge needles do not tend to clog when being used to administer drug solutions through the skin, but cause the patient significant additional pain or discomfort when entering the skin.
A variety of devices have been proposed for the subcutaneous administration of drug solutions, formulations, and suspensions. One type of device utilizes a dual-compartment syringe. One compartment of the syringe contains a diluent, and the other compartment contains a powdered drug. The sidewall of the syringe contains a groove just forward of the stopper between the chambers. As the plunger is pushed, the groove allows fluid to leak into the drug chamber. The drug and diluent are mixed by the turbulence created as the fluid from the diluent chamber enters the drug chamber, and then the injection is administered through an attached syringe. The disadvantages of this method include use of a non-custom syringe and a large needle which needs to accommodate the drug/diluent mixture, thereby causing excessive pain in the patient.
Devices which include a sheath wrapped around a syringe needle have also been used. However, to the extent that the sheath may be made of a polymer, the sheath does not dilate an injection area once the needle is inserted into skin. In these devices the sheaths protect the needle during injection and during the withdrawal of the needle after injection. For example, a common application involves a sheath which is tightly wrapped over a steel needle. The needle is inserted into the skin, and once in place, the needle is withdrawn from the skin, leaving the sheath in a desired position within the skin. After this a drug delivery device is used to inject fluid through the sheath. But these devices present difficulties to users who usually must perform injections with only one hand available to manipulate the device.
Thus a need exists for injection devices that allow for the administration of substances of higher viscosity while minimizing patient discomfort.